The present invention relates to a thin type battery using a laminated sheathing and particularly to a thin type battery with a sealing part having a novel structure.
A thin type battery with a laminated sheathing is shown in the plan view of FIG. 1 and in the cross sectional view of FIG. 2. The thin type battery shown in these figures has an electricity generating element 7 housed in the laminated sheathing 5. An electrolytic layer 3 is interposed between a positive electrode 1 and a negative electrode 2 of the electricity generating element 7. Adjacent an outer circumference of the electricity generating element 7 of the thin type battery is a sealing part 4 of the laminated sheathing 5. The laminated sheathing 5 airtightly seals the electricity generating element 7 and includes two laminated films 5A on both sides of the electricity generating element 7. The outer circumference of the films 5A are heat welded, and at that time, collector terminals 6, which are electrically connected to the positive electrode 1 and to the negative electrode 2, are pulled outwardly of the laminated sheathing 5. Note, that it is important that the laminated sheathing 5 airtightly seals the electricity generating element 7. In particular, it is important that the seal is airtight to prevent the introduction of atmospheric moisture inside the sealed battery. The following items are important in preventing the introduction of moisture inside the laminated sheathing 5.
1) A laminated film with a low moisture transmission rate must be used. PA1 2) The seal must prevent the introduction of moisture.
To lower moisture transmission of the laminated film 5A, the laminated film 5A is composed of a polyester film 5a which is laminated on one face of an aluminum layer 5b and of a polypropylene film 5c which is laminated on the other side. In comparison to a plastic film, the aluminum layer 5b has an excellent property of limiting the transmission rate of moisture. However, because the aluminum layer 5b cannot be heat welded, the polypropylene film 5c, which can be heat welded, is laminated on the inner face of the aluminum layer 5b. Also, to protect the aluminum layer 5b, the polyester film 5a is laminated on the outer face of the aluminum layer 5b.
To prevent the introduction of moisture through the sealing part 4, the laminated sheathing needs to augment the width of the sealing part 4. As shown in the cross-sectional view of FIG. 3, the width of the sealing part 4 is narrow, and thus moisture can enter the inside of the battery by passing through the polypropylene film 5c that is laminated on the inner face of the aluminum layer 5b. Thus, the sealing part 4 is a structure that cannot prevent moisture transmission by the aluminum layer 5b. Since the polypropylene films 5c are crimped or sandwiched between the aluminum layers 5b, moisture is able to penetrate the polypropylene films 5c. In comparison to the aluminum layer 5b, the polypropylene film 5c has a fairly large moisture transmission rate. For this reason, the width of heat weld formed by the polypropylene film 5c is made fairly large. In other words, in order to reduce moisture transmission, it is necessary lengthen the distance that the moisture will have to travel the seal. For this reason, it is not possible to have a laminated sheathing 5 with a narrow sealing part 4.
Accordingly, the thin type battery which has a wide sealing part at the outer circumference of the electricity generating element, has an electricity generating element which is small compared to the external size of the battery. For this reason, the drawback is that the electric discharge capacity of the battery is reduced. However, if the width of the sealing part is made narrower, the moisture etc., can more easily penetrate interiorly of the battery by passing through the polypropylene films of the sealed portion.
The present invention has been developed in order to solve these drawbacks. The important object of the present invention is to improve the seal provided by the sealing part of the laminated sheathing and also to provide a thin type battery with a laminated sheathing which has a small overall outer size and a large electric discharge capacity.